The present invention relates to a method of producing a metal foam, wherein to foam a metal, the size of the bubbles to be created within the bulk or on the surface of a foamable metal in the liquid phase is controlled, preferentially decreased or increased, and in particular set to a given value by means of applying oscillations simultaneously with the formation of said bubbles. The invention also relates to the metal foam product produced by the method in accordance with the invention.
Metal foams are closed-cell or open-cell constructional materials with light weight and exhibit outstanding physical, mechanical, as well as heat and acoustical insulation properties. Due to their properties, metal foams can be applied in a broad range extending from constructional materials to various ornaments. Mainly due to their low densities and extraordinary mechanical properties, some exemplary fields of use of metal foams include e.g. aircraft industry (wings, seats), space technology (heat insulation, vibration damping), automotive industry (various bumping elements, body parts, acoustical cushionings), defence industry (screenings within the radio frequency domain, strong armours), medical applications (such as osteoprostheses), as well as construction industry (frame constructions, light weight construction walls), and others.
Several techniques are known for the production of metal foams. In the most common way, a metal foam is prepared by adding a gas forming additive (such as hydrides, carbonates, and the like) to the melt of a metal also comprising stabilizing grains/particles. According to our present knowledge, due to the uncontrollability of the undergoing chemical and physical reactions, no metal foam of a monomodal pore size distribution can be produced by means of introducing gas forming additives. Another technique for producing a metal foam is to introduce gas bubbles into the melt of a metal. In general, said supply of gas bubbles is effected by means of either an agitating device (see e.g. Japanese Laid-open Patent Publication No. 2006-176874) or a suitable injection means (see e.g. U.S. Patent Application No. 20030047036).
German Patent Application No. DE 43 05 660 A1 discloses a method and an apparatus for adjusting the size of the bubbles of a bubbled medium in the liquid phase, wherein said adjustment takes place subsequently, that is, after the creation of said bubbles. The core of said method is to expose the bubbles formed in the medium by e.g. injecting a gas through a nozzle orifice to ultrasonic waves after they had detached from the nozzle and left the nozzle end; the bubbles are formed at the nozzle end with a diameter that is defined by the geometrical parameters of the nozzle end in combination with further parameters influencing the way of the injection (such as the gas flow rate and the gas injection pressure). The frequency and power of the applied ultrasonic waves are chosen in such a manner that due to the exposition the bubbles get into resonance, and as a result of that they burst into bubbles of smaller size.
European Patent Application No. 0680779 A1 teaches a method for facilitating the dissolution of a gas into a liquid kept in flowing. In said method, bubbles formed previously in the liquid by means of injecting gas through a nozzle are brought into interaction with ultrasonic waves. As a result, the bubbles within the liquid are excited to frequencies beyond the resonant frequency and then split into bubbles of smaller size due to the energy absorbed from the ultrasonic waves. The thus obtained smaller bubbles rapidly dissolve into the liquid.
Japanese Laid-open Patent Publication No. 57-165160 discloses the fabrication process of a metal tape with a porous and amorphous structure, as well as an apparatus for accomplishing said fabrication process. According to the solution taught, a molten metal is put into a closed crucible and is then injected via a nozzle onto a cooling roll that rotates at a high speed by means of exerting a pressure onto the melt within the crucible by a gas introduced into the crucible via an inlet pipe. Simultaneously, gaseous nitrogen, air or another inert gas is also blown via a gas blowing pipe into the molten metal and to said nozzle, thereby forming bubbles in the melt in the vicinity of the nozzle end. Thereafter, the bubbles detached from said nozzle are dispersed by means of mechanical vibrations generated by a high frequency generator and/or further bubbles are formed by the cavitation effect within the molten metal from the gas dissolved into said melt. Thereby, a relatively fine distribution of bubbles is achieved within the melt. The bubbled molten metal solidifies on the surface of the rotating roll very rapidly and forms an amorphous thin metal foam tape thereon.
Said fabrication process is solely suitable for the preparation of thin metal foam tapes spreading essentially in-plane and cannot be used to produce a liquid or a solid metal foam in the shape of a block (i.e. with three well-defined spatial dimensions).
A common drawback of the above discussed solutions is that they all aim at decreasing the size of previously formed bubbles in a liquid medium by means of breaking up said bubbles with ultrasonic waves. As such a technique is based on bringing into vibration the already existing bubbles by the ultrasonic waves and thereby inducing spatial deformation thereof and then tearing said bubbles into parts of smaller size at an extreme value of the deformation, this technique is not suitable for performing the break-up of the gas bubbles in a controlled manner: typically, the size of the bubbles obtained through break-up varies in a broad range, and thus, the pore size distribution of the metal foam obtained by this technique also varies between wide limits. Hence, the formation of bubbles with about the same size cannot be ensured by such a technique. Neither can be ensured the production of a metal foam with a (nearly) monomodal pore size distribution. The high frequency generator applied affects the size of the bubbles detached from the nozzle end in an uncontrollable way; the ultrasonic waves emitted by said generator will decrease the size of those bubbles of the metal foam in the liquid phase that had already previously formed.
A method and a capillary system for producing a liquid metal foam with a monomodal size distribution (here, the pore size being greater than 3 mm) from a molten metal are disclosed in European Patent No. 1419835 B1. The desired size of the pores of the liquid metal foam being formed is essentially accomplished by dimensioning the individual nozzles of the capillary system, as well as by appropriately choosing the relative distance of said nozzles and the geometrical design of the nozzle ends. Consequently, only a metal foam of a certain pore size can be obtained by a capillary system of a given geometry—the preparation of metal foams of different pore sizes requires the application of capillary systems of several geometries.
A further common drawback of the above solutions is that a gas injection via one or more nozzles, used as a tool to create the bubbles, forms an essential element thereof.